The present invention relates to direction finding and, more particularly, to a method and apparatus for identifying and tracking a target in a noisy environment.
Amir Averbuch, et al., in xe2x80x9cA wavelet packet algorithm for classification and detection of moving vehiclesxe2x80x9d, Multidimensional Systems and Signal Processing vol. 12 pp. 9-31 (2001), which is incorporated by reference for all purposes as if fully set forth herein, describe a method of detecting the presence of a member of a class of acoustic emitters (for example, a vehicle of a particular type) on the basis of an analysis of the acoustic signature of the class member. A wavelet transform is applied to several overlapping portions, each of length n, of a received acoustic signal, up to a preselected level m. Each transform produces mn transform coefficients arranged in 2m+1xe2x88x921 blocks associated with different frequency bands. Only the blocks known to be associated with a signature of the target class are retained for further processing. The energies of these blocks are computed and compared to the energies of these blocks that were previously measured for representative members of both the target class and other classes from which members of the target class are to be discriminated. Each transform thus is associated with one of the classes for which signature block energies have been measured. The acoustic emitter is assigned to the class with which the most transforms are associated. The measure of the confidence with which the acoustic emitter has been identified is the ratio of the number of transforms associated with the winning class to the total number of transforms.
The present invention is an extension of the methodology of Averbuch et al. to direction finding. For example, the present invention can be used to identify and track enemy combat vehicles such as airplanes and helicopters.
According to the present invention there is provided a method of tracking a target, including the steps of: (a) receiving a signal; (b) identifying, in the signal, a signature of the target; (c) determining, from the signal, a direction to the target; and (d) iterating the identifying and the determining, each identifying after a first the identifying being based on an immediately preceding determining.
According to the present invention there is provided a method of tracking a target, including the steps of: (a) receiving a signal; (b) identifying, in the signal, a signature of the target; (c) determining, from the signal, a direction to the target; and (d) iterating the identifying and the determining, each determining being based on an immediately preceding identifying.
According to the present invention there is provided an apparatus for tracking a target, including: (a) a receiver for receiving a signal; and (b) a processor for: (i) identifying, in the signal, a signature of the target, (ii) determining, from the signal, a direction to the target, and (iii) iterating the identifying and the determining, each identifying after a first identifying being based on an immediately preceding determining.
According to the present invention there is provided an apparatus for tracking a target, including: (a) a receiver for receiving a signal; and (b) a processor for: (i) identifying, in the signal, a signature of the target, (ii) determining, from the signal, a direction to the target, and (iii) iterating the identifying and the determining, each determining being based on an immediately preceding identifying.
According to the present invention there is provided an apparatus for receiving a signal, including: (a) an antenna, having an aperture, for receiving a plurality of representations of the signal; and (b) a processor for processing the representations in a manner that gives the antenna an effective beamwidth at least twenty percent smaller than a reciprocal of the aperture.
The present invention is directed towards the identification and tracking of an emitter of any kind of wave energy, for example electromagnetic energy. Nevertheless, the examples herein relate to the identification and tracking of a target that emits acoustic energy. Correspondingly, the term xe2x80x9cmicrophonexe2x80x9d as used herein is to be understood as referring to the special case of an antenna element for receiving acoustic signals. The target may be stationary or moving.
The point of innovation of the present invention is the iterative alteration of identification and direction finding, with the results of each identification step being used as a basis for the immediately subsequent direction finding step, and with the results of each direction finding step being used as a basis for the immediately subsequent identification step. Specifically, the results of each identification step are used to improve the immediately subsequent direction finding step, and the results of each direction finding step are used to improve the immediately subsequent identification step. The iteration continues until a termination criterion is satisfied. One such termination criterion is that the target is identified at a confidence level exceeding a predetermined threshold. Another such termination criterion is that the direction finding converges.
As described above, the identification method of Averbuch et al. includes applying a discrete transform, specifically a wavelet transform, to sections of a received signal. For identification only, the signal could be received using an omnidirectional antenna. For the purpose of combined identification and direction finding, however, the signal is received using a directional antenna, for example a phased array antenna composed of a plurality of antenna elements. The antenna elements could be arranged, for example, in a linear array or in a circular array. xe2x80x9cReceivingxe2x80x9d the signal is to be understood as including digitizing the signal as received by the antenna elements to provide, for each antenna element, a corresponding set of signal samples. It is these signal samples, or subsets thereof, that are input to the discrete transform.
Preferably, the iterative alteration between identification and direction finding relies on using the same discrete transform for both identification and direction finding. Direction finding is effected by finding the direction that maximizes the energy of a weighted sum of the transform coefficients output from the discrete transform in each direction finding step. Preferably, the only transform coefficients that enter this weighted sum are transform coefficients whose basis functions are associated with the target signature that is used in the identification steps. The direction found in each direction finding step then is used in the subsequent identification step, to form, for each basis function associated with the target signature, a direction-weighted sum of the corresponding transform coefficients output from the discrete transform in that identification step.
Although the scope of the present invention includes any suitable discrete transform, for example a discrete Fourier transform, the preferred discrete transform is a wavelet transform. The most preferred discrete transform is a wavelet transform whose basis functions are eighth-order spline wavelet packet functions and in which the wavelet decompositions are continued to seventh order.
An apparatus of the present invention includes a receiver for receiving the signal and a processor for implementing the iterative alteration between identification and direction finding. The receiver includes an antenna, as described above. Using a wavelet transform as the discrete transform allows the antenna to have an effective beamwidth that is smaller than the reciprocal of the antenna aperture by twenty percent, or even by thirty percent.